We present surprising experimental evidence regarding the past of photons passing through an interferometer. The information about the positions through which the photons pass in the interferometer is retrieved from modulations of the detected signal at the vibration frequencies of mirrors the photons bounce off. From the analysis we conclude that the past of the photons is not represented by continuous trajectories, although a "common sense" analysis adopted in various welcher weg measurements, delayed-choice which-path experiments, and counterfactual communication demonstrations yields a single trajectory. The experimental results have a simple explanation in the framework of the two-state vector formalism of quantum theory.

We report the experimental observation of exciton spin relaxation in GaAs quantum wells in moderate magnetic fields. We resolve the electron and hole contributions and discuss the large sensitivity of the spin-relaxation time to exciton localization and quantum well width. We use the long duration of spin orientation to demonstrate deep transient oscillations, resulting from biexcitonic effects. PACS numbers: 73.20.Dx, 42.50.Md, 71.35,+z, 78.65.FaThe spin-relaxation process in semiconductor heterostructures results in depolarization of the photoluminescence (PL) and of the induced absorption, as well as in dephasing of the electronic wave function. It is believed that this process is substantially modified in twodimensional systems, especially for holes whose degeneracy at the zone center is removed. While there is a vast literature on the energy relaxation process of electrons and holes in semiconductor quantum wells (QW), there are only a few investigations of carrier spin relaxation. The determination of the dynamics of this process and the understanding of the mechanisms which are involved have been of growing interest in recent years [1-1 ll. Optical spectroscopy has been extensively implemented for the research of spin-relaxation processes in GaAs QW, mainly by way of measuring the degree of polarization in the PL emitted following cw excitation by polarized light [1,2]. The electron spin-relaxation time, deduced with the assumption of instantaneous hole spin relaxation during thermalization [3], was found to be -200 ps at 4 K. This is not very different from the spin-relaxation times reported for bulk GaAs and Al-GaAs [4]. Recent PL excitation experiments in QW in magnetic fields up to 20 T claimed inhibition of the spin flip of conduction electrons during thermalization [5].Time-resolved optical spectroscopy allows separation between the initial depolarization, resulting from the thermalization process, and that of low-energy carriers at the bottom of the band. Correlating time-resolved and cw PL spectroscopy, Freeman et al. have shown that spin lifetimes cannot reliably be extracted from cw spectra [6]. They have also found that the depolarization rate is sample dependent. Using the same technique and exciting a high-quality QW sample at the excitonic resonance, Damen et al. reported an electron spin-relaxation time constant of ~ 50 ps and instantaneous spin relaxation for the holes [7]. Theoretical studies, however, predict a significant suppression of the spin flip rate for free heavy-hole states near the zone center [8-10]. This apparent disagreement between theory and experiment indicates that the process of spin relaxation of excitons is not well understood and further experimental data are required.Time-resolved absorption measurements have the po-

We present investigations of fs time resolved coherent wave mixing under high magnetic field. Our experiments reveal a new regime at high magnetic field and low excitation density dominated by the Coulomb interaction. This regime is inconsistent with the semiconductor Bloch equations. A model which includes exciton-exciton correlation successfully describes many features of this regime.[S0031-9007(97)